When Humans, Chimps Were Kissin' Cousins
DAVID PERLMAN
San Francisco Chronicle, 05/18/2006
CORRECTION: -- A story Thursday on human evolution misstated the location of X
chromosomes. They are carried by both male and female cells. (05/19/06, P. A2)
The story of human evolution that began when the earliest ancestors of humans
and chimpanzees first separated from their common ancestor has taken a new
turn with a claim that the epochal event may well have occurred millions of
years more recently than the fossil record has long maintained.
On top of that, the split of the two lineages was probably far more complex
than anyone had thought -- with some of the earliest members of both lines
interbreeding again and again before the two species finally separated
permanently, according to scientists analyzing the genes of both modern chimps
and modern humans.
If that controversial claim turns out to be true, the famed fossil skull of
the creature nicknamed Toumai -- and dated at roughly 7 million years old --
may have lived long before the final chimp-human split occurred, say
researchers at the Broad Institute, a collaborative venture linking the
Massachusetts Institute of Technology and Harvard.
Toumai (Sahelanthropus tchadensis), discovered five years ago in the desert of
Chad by French anthropologists, has until now been considered the earliest
known hominid representative in the human lineages that split from a common
ancestor of humans and chimps.
But by analyzing the genes of modern humans and chimps and other primates, the
MIT and Harvard researchers say, the true split between the ancestral ape and
human lineages could have begun much later -- probably much less than 5.4
million years ago and surely no earlier than 6.3 million years ago -- than
previous estimates based on the fossil record.
The new estimate for the earliest beginnings of human evolution was published
online Wednesday in the journal Nature by a group led by David Reich, a
geneticist at the Broad Institute, in Cambridge, Mass. The group includes
famed geneticist Eric Lander, a leader of the Human Genome Project and the
Broad Institute's director.
The scientists analyzed the genes in the X chromosomes of humans, chimps,
gorillas, orangutan and macaques -- the chromosomes carried only by the male
lines of those animals. They noted that creatures like Toumai, which displayed
both human and chimp features and has been set at between 6.5 million and 7.4
million years old, could well have come from a species whose members were
interbreeding before the final separation between the lineages of human and
chimp species occurred.
"The study gave unexpected results about how we separated from our closest
relatives, the chimpanzees," Reich said in a statement. "We found that the
population structure that existed around the time of human-chimpanzee
speciation was unlike any modern ape population. Something very unusual
happened at the time of speciation."
The unusual happening was that once the two species began separating from
their common ancestors, they might well have been repeatedly interbreeding
with each other -- at times producing sterile offspring but sometimes
producing offspring that remained fertile and, in turn, created a short-lived
mixed lineage -- a kind of dead-end tribe combining both pre-human and pre-
chimp genes.
It was one of those interbred hybrid groups that ultimately began the human
lineage, according to the Broad Institute group.
"It's a fascinating tale of ancient shenanigans," said David Haussler, a
leading geneticist at UC Santa Cruz in an e-mail message. "I continue to be
amazed at what DNA analysis can reveal."
Edward M. Rubin, director of the Department of Energy's Joint Genome Institute
and chief of genetics at the Lawrence Berkeley Laboratory, was equally
intrigued.
Rubin, who has been interested in determining the genetic relationship between
Neanderthals and modern humans, called work of the Reich team "complicated,
but plausible."
"Until now our view of human evolution has largely come from the fossil record
which is still incomplete," he said. "This is very provocative because it
provides a new window into a time when hominids separated from the common
ancestor of chimpanzees. It's the beginning of a new wave of work that will
bring us new insights into our view of how species separate."
Rubin noted that the Broad Institute team worked with only 20 million base
pairs of genes in their genetic analysis of the X chromosomes of humans and
the other primates, while some 3 billion base pairs are available in the human
genome alone. Even firmer conclusions will come, he said, when the entire
genome of the gorilla has been determined, as this will provide evidence for
the far more ancient time when the gorilla and chimp lineages separated.
At the Broad Institute, Nick Patterson, a statistical geneticist, noted
that "if the (fossil) dating is correct, the Toumai fossil would precede the
human-chimp split. The fact that it has human- chimp features suggests that
human-chimp speciation may have occurred over a long period of time with
episodes of hybridization between the emerging species."